Twicx's Dual E3D Direct-Drive Extruder

Discussion in 'E3D-v6 and Lite6' started by twicx, Mar 6, 2014.

  1. Chavaquiah

    Chavaquiah Active Member

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    I thought as much.

    Do you have any PCB etching equipment? Unless you take out a lot of copper so as to create one (or several) long track, the resistance will be almost nil. Any connection there will create a short-circuit that will probably fry the electronics even before the fuse has time to react.
     
  2. twicx

    twicx Well-Known Member

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    I don't, but I could probably still do it. I did it before using a stanley and a lot of patience.
     
  3. Chavaquiah

    Chavaquiah Active Member

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    In that case, that's pretty much how PCB bed heaters are made.

    Do you have any reference to the copper thickness on that board? It might be something like 35µm. With that number you might use a trace width calculator(*) to have an idea for a good track size.

    (*) Example: http://circuitcalculator.com/wordpress/ ... alculator/


    EDIT:

    On a different note, and since you intend to use a glass, how about having the heater right under the glass? With close enough tracks, heat distribution shouldn't be a problem and the less weight for the bed, the faster you can print.
     
  4. twicx

    twicx Well-Known Member

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    yeah that's exactly how thick it is haha!
     
  5. twicx

    twicx Well-Known Member

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    that calculator confuses me :/ I'm trying to work out length/width for a temperature of 110?
     
  6. Chavaquiah

    Chavaquiah Active Member

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    Remind me: what is the intended dimension for the bed?
     
  7. twicx

    twicx Well-Known Member

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    VERY APPROXIMATELY 275 x 295. that's probably +/- 10mm each side?
     
  8. Eaglezsoar

    Eaglezsoar Administrator

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    How do you propose to put traces on it?
     
  9. twicx

    twicx Well-Known Member

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    I used to make guitar amps and pedals, and when I was stuck for stripboard, or I was making a really small circuit, I'd trace out the circuit with permanent marker and cut out the excess copper with a steel rule and a stanley and a tonne of patience. Not ideal, I know, but it'll have to do. I just need to figure out how long/wide to make them.
     
  10. twicx

    twicx Well-Known Member

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    [​IMG]

    So 0.4R is the resistance across pretty much any 2 points on the copper. Am I correct in saying that if the board was actually traced out to have a number of paths that were long, I'd get higher resistances? If so, what should I be aiming for?
     
  11. Chavaquiah

    Chavaquiah Active Member

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    The resistance you're measuring now comes more from the probe's tips to board connection and leads. You might record this value (that seems a tad high) and deduce it from further measurements, but it's probably not worth it.

    In the end, whatever the configuration, you must have a resistance as low as possible (more power going to heat) BUT that is at least enough to limit the current to something both your PSU can supply and your electronics can handle.

    With a 21A PSU, and reserving at least 5A for motors and such, you could have, theoretically, at most 16A for the bed. That would leave no safety margin whatsoever and any overvoltage situation would cause a lot a trouble. I would not consider more than about 12A.

    Working with Ohm's Law, you want a resistance that is at least:

    R >= I / V ==> R >= 12A / 12V ==> R >= 1Ω

    I seem to remember you wrote that the boards you got are slightly larger than needed. Perhaps you could start by cutting out the excess and using the scrap as a test board. Draw a moderately long (20cm or so) track about 2mm wide and measure it's resistance. That might be a good base to start from.
     
  12. twicx

    twicx Well-Known Member

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    Yeah I think that's a good plan. It should leave me enough for the new motor and hot end too. Hopefully it'll give me enough heat
     
  13. Eaglezsoar

    Eaglezsoar Administrator

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    On the heatbed for the Rostock Max, there is a total resistance of approximately 1.5 ohms. All I can tell you about it is that it
    has hundreds of traces on it to achieve the low resistance. You are going to have a rough time doing something like that by hand.
     
  14. Chavaquiah

    Chavaquiah Active Member

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    Sort of an understatement, really.

    I would get some ferric chloride (remember protection gloves) and an etch resist pen and do it the more traditional way. Not that I ever tried it, though.
     
  15. twicx

    twicx Well-Known Member

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    Ok so some good testing done today! I didn't get much time to work on it, but in 40 minutes, I managed to trace up one of the cut off pieces. Its 65x305mm, with a series of 5mm tracks, all connected. 0.9R for just that piece. So by my estimations, I'll come out with a resistance for the bed between 2R7 and 3R5. That sounds like a nice value I think?
     
  16. Chavaquiah

    Chavaquiah Active Member

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    Maybe not so nice. With a total R of 2R7 your bed would generate about 53W. Doesn't seem to be enough.

    However, if you have 4 segments of 0R9, you could organize them as 2 parallel sets of 2 serial segments, for a grand total of... wait for it!... 0.9 ohms:

    Code:
                 +------- 0R9 ------- 0R9 -------+
                 |                               |
    12V ---------+                               +-------- GND
                 |                               |
                 +------- 0R9 ------- 0R9 -------+
    
     
  17. twicx

    twicx Well-Known Member

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    Ok, so I'm making a little bit of progress today. I now have the two pieces cut to size, and the mounting holes marked out. I know it looks like a lot, but i might aswell use whats already there.
    http://imgur.com/StlSIfJ.jpg

    So I still need to decide what way to divide them up. I think 4 sections (2 per piece) and do them as suggested above sounds good, but I was also thinking that if i just make each side 1.8R, I'd be getting the same result, right? and it would be easier.

    Also, I got a bunch of these thermistors with my RUMBA board. I don't know what kind they are, or the best way to mount them, but if I'm upgrading the heater, it seems like as good a time as any to replace the thermistor, right?
    http://imgur.com/owqEd8B.jpg
     
  18. Eaglezsoar

    Eaglezsoar Administrator

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    Separating into 4 sections sounds like a good way to do it. Whats important is the final resistance.
    The thermistor could be problematic if you don't know it's beta value, but there are ways to figure it out
    Go for it! I can't wait to see the final product working.
     
  19. twicx

    twicx Well-Known Member

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    So 4 sections are better than 2? 4 legs good, 2 legs bad? Lol!

    The thermistors are 100K ohm EPCOS B57560G104F NTC Thermistors. They come with rumba kits from the vast majority of resellers. I also have a sneaky suspicion that they're one of the presets in the marlin files for the rumba.
     
  20. Eaglezsoar

    Eaglezsoar Administrator

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    Knowing the part number of the thermistors will help a lot. I do believe 4 sections are better than 2, more even heat distribution.
     

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